Knowledge of the precise distortion characteristics of a particular projection lens system ("lens") is required in a variety of optics applications such as in VLSI and ULSI photolithographic technology. A lens with unwanted distortion could cause defects in IC chips and thus lower the yield per wafer and directly affect the profit margin of a manufacturer. It is to be understood that such projection lens systems are optically complex, including a large number of lens elements, and so their optical properties may be difficult to determine from purely theoretical considerations. The amount of distortion which will cause unwanted defects decreases as process technology provides for smaller IC feature dimensions and greater integration. Generally, the larger the lens, the more opportunity for the inadvertent introduction of distortion. Therefore, distortion measurement is especially necessary where precision is required of larger lenses in LSI, VLSI, and ULSI processes.
Not all distortion is necessarily unwanted. Lens distortion measurement systems may be used to measure a lens in which the desired distortion is zero or a lens in which the desired distortion is nonzero (as in a telescope or microscope).
In a first prior art projection lens distortion measurement system for photolithographic lenses, an exposed mark is first formed on a semiconductor wafer in a predetermined position. The wafer is then loaded into a stepper. The illumination which defines the image of the mark passes through the lens to be measured. A sensor on the side of the lens remote from the illumination source detects the position of the mark in the transmitted image. The system then compares the actual position of the mark image received at the sensor to the expected position of the mark image if the desired distortion was present. The undesirable distortion can then be determined based on the offset of the mark. One commercially available software package which facilitates this process is called "AMS" software developed by Nikon Corporation.
This system requires fabricating a mark on a wafer thereby utilizing valuable equipment time and space. Furthermore, this system allows for measurement of only one position in the lens field at a time. Typically, distortion measurement of the entire field of the lens is desired. Therefore a lens distortion measurement system which can determine the distortion of the whole field of a lens without requiring process steps for forming a mark on a wafer was desired.
In a second prior art lens distortion measurement system, a mark is fabricated on a wafer. An additional machine for moving the wafer laterally with respect to the lens is used to move the wafer so that the mark image passes through a representative sampling of the entire field of the lens. This machine must be precise in its movements so that the actual location of the mark can be determined. One such commercially available machine is the Nikon "XY" system developed by Nikon Corporation. The process described above for the first prior art lens distortion measurement system is repeated for each point in the representative sample. Interpolation of the representative sample distortion measurements can be performed to obtain reasonable estimations of the distortion for points in the field that were not directly measured.
While this system allows for distortion measurement of the entire field of a lens, it still requires that a wafer be fabricated with a mark. Furthermore, this system requires the use of the additional machine which costs money and utilizes valuable chip fab real estate. Even though the machine is precise, it can introduce significant error if the actual movement of the wafer does not exactly match the expected movement, thereby presenting the system with a false position of the mark and thus false distortion measurement. This distortion is called "stage accuracy" distortion and is currently the most limiting factor in obtaining an accurate distortion measurement.
Therefore, what is desired is a lens distortion measurement system which determines the distortion of the entire field of a lens without requiring a mark on a wafer and without requiring an additional machine that introduces stage inaccuracies.